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Welcome to the USGS Trek Through Time, where you can walk part of geologic history! The Trek is on the campus of USGS headquarters in Reston, VA. If you're nearby, park for free in the visitor's lot, and head to the left corner of the building. Walk down a small ramp, and enjoy a stroll in the woods. If you aren't, you can do a virtual walk by following these links.


The Trek Through Time was conceived and brought to life by Laurel Bybell, USGS Scientist Emerita. She produced it three times for events and finally in 2020 as a permanent installation.

Others who helped realize the final version: Ellen Seefelt (design of plaques), David Govoni, (paleontology, editing), USGS OSQI staff (funding, administrative support, website), Pannier (signs), Berntsen Trail Posts (age markers), and Nealon Property Management (installation).

How old is Earth?

Earth is really, really old…about 4.6 billion years old! A billion is equal to 1000 million, and numbers that large can be challenging to think about. If you tried to count to these numbers without stopping, here’s how long it would take:

1 million: approximately 11 days

1 billion: approximately 31 years   

4.6 billion: approximately 143 years!

How was the Geologic Time Scale made?

Relative Geologic Time

  • Relative time places rocks in order relative to each other.
  • In the late 1700s, geologists noticed that layered rocks always appeared in the same order, and that fossils in deeper rocks were always more primitive. They concluded, logically, that younger rocks pile up on top of older rocks.  
  • They used the changes in plant and animal fossils over time and the Principles of Stratigraphy to determine the relative ages of rocks.

Absolute Geologic Time

  • Absolute time assigns ages to rocks in millions of years before present.
  • Absolute ages are discovered through chemistry, and the invention of a tool called the mass spectrometer in the 1900s advanced modern dating techniques, providing the ability to measure the ratios of radioactive isotopes (elements with the same number of protons but a different # of neutrons, which decay over time) in rocks.
  • Because the half-life, or how long it takes for half of a radioactive isotope to decay, is known, the age of the rock can be determined.
  • Scientists now agree that Earth is approximately 4.6 billion years old.
  • The divisions of the geologic timescale are updated every few years as new evidence and new technologies improve the precision of the dates.
  • So, these signposts are accurate in the year 2020, but may need to be updated every few years as we learn even more!


KIDS’ Corner: What’s an element? Elements are very small, unique substances with a specific number of protons (+ charges) and electrons (- charges), and a variable number of neutrons (no charge). Isotopes of elements have the same number of protons, but a different number of neutrons, which changes their atomic weight. For example, all carbon atoms have 6 protons - that is the definition of carbon. Some have 6 neutrons (12C), some 7 (13C), and some 8 (14C). Can you figure out what the 12, 13, and 14 come from in this example?*

Isotopes can be stable (do not decay over time, like 12C and 13C) or radioactive (decay over time, like 14C). There are over 100 naturally occurring elements, and each element can have one or more isotopes. For more information about isotopes and the Periodic Table of the Elements, visit:

*answer: the sum of the number of protons plus the number of neutrons. We say out-loud "Carbon-14" when we read 14C.

Today’s walk along the Trek Through Time

  • You’re about to walk through a small part of geologic time as seen in the green, blue, and yellow sections of the diagram below. The signposts along the walk match these colors.
  • This walk is through the Phanerozoic Eon, or the most recent 541 million years of geologic time, where fossil evidence of complex, abundant life is found.
  • Life was much more primitive before the Phanerozoic Eon (the more than 4 billion years known as the Precambrian, and shown in black below) and is not represented in the Trek Through Time.
Simplified time scale showing the relative length of the Phanerozoic with respect to geologic time.
This simplified geologic timescale shows the length time (in black) before life on Earth shows up in abundance in the fossil record. The Phanerozoic is represented on the right end of line in proportional colored blocks, and is subdivided into the Paleozoic (colored green), Mesozoic (blue), and Cenozoic (yellow) Eras. 

The Phanerozoic Eon is divided into three Eras:

U.S. Tapestry of Time and Terrain
U.S. Tapestry of Time and Terrain 
  • The Paleozoic Era - green signposts
  • The Mesozoic Era - blue signposts
  • The Cenozoic Era - yellow signposts
  • Each Era is split into even smaller parts known as Periods and Epochs. These boundaries were first defined by the appearance (evolution) or disappearance (extinction) of fossil species in rocks. That is why mass extinctions commonly define the boundaries between geologic ages.

  • The colors for the signposts were chosen because they match the international color scheme for geologic maps, and for the geoogic time scale: Paleozoic rocks are in greens, Mesozoic in blues, and Cenozoic in yellows.

  • Tapestry of Time and Terrain is a USGS geologic map of the United States. You can see the colors of the timescale on the right, with the youngest at the top, just like the rock layers themselves. You may notice that in this map, the Paleozoic rocks are blue and the Mesozoic rocks are green. Why does this map differ from the international convention? Well the USGS has used this color convention since the 1800's. In 2006, the national and international groups that decide on these conventions came to an agreement that large scale and regional maps of United States will use the USGS colors, and international maps, plus smaller US and North American maps will use the international colors. 


Let’s get walking! (click on this link or start with Paleozoic above)

  • Every 3.6 feet you will walk today are equal to one million years of time, so the entire trek of 541 million years will be about 1/3 of a mile.
  • You will see a signpost every 10 million years and will move forward in time until you reach the “0” signpost that means “0 millions of years before present” or more simply, TODAY!
  • Notice how life on Earth has changed over time and how the location of present-day Virginia has moved! This is called paleogeography, and this information is pieced together by the understanding of plate tectonics.
    • Plate Tectonics: The study of how the rigid, outer surface of Earth (known as tectonic plates) interact (move toward each other, away from each other, or slide past each other). The plates move about as fast as your fingernails grow.
  • If you wanted to make a Trek Through Time starting from the formation of the Earth, your 4.6 billion-year walk would be more than three miles long!